tridacna(n = noun.animal) genus tridacna - type genus of the family Tridacnidae: giant clams;
is a kind of mollusk genus
is a member of family tridacnidae, tridacnidae

   A genus of very large marine bivalve shells found on the coral reefs of the Indian and Pacific Oceans. One species (Tridacna gigas) often weighs four or five hundred pounds, and is sometimes used for baptismal fonts. Called also paw shell, and fountain shell. [1913 Webster]

genus tridacna, tridacna gigas